The automotive, aeronautical, aerospatial, nuclear and other mechanical manufacturing industries generally use screwed assemblies and this in applications that are extremely varied and complex.
The calculation notes for screwed assemblies are aimed at determining the prestress to apply to the assembly as a function of the useful stress (or service stress) to which the assembly will be subjected when used and of a tightening factor depending on the precision of the means for applying this prestress.
These calculation notes done by engineers studying in these industries are very precise as a function of the defined needs and the conditions of use of the screwed assemblies are more and more severe so that it is getting essential to increase the reliability of these assemblies while respecting the calculation notes in their execution so as to increase the performances and the safety of the systems thus made.
The stress to apply in a threaded mounting element is generally created by conversion of a rotation force into a tractive force by the helicoidal path of the thread of the element. This conversion is imperfect mainly because of the losses from friction of the surfaces in contact; hence it is necessary to apply a rotation force (torque) greater than the rotation force necessary for this conversion.
The efficiency of this conversion is extremely varied because of the variation of the coefficients of friction, the variation of the distance to which the resultant of the friction forces are applied relative to the axis of rotation, and the geometric variation of the parts, mainly the contacting surfaces. Practical observations give a variation of 50% of the induced stress during application of a constant torque on the same lot of mounting elements. It is verified that the improvement of the precision of the applied torque does not bring about a significant improvement in the precision of the tension measured.
A first improvement of the variation of the stress generated by the torque has been associated with the rotation angle starting from a predetermined torque. In effect the lengthening of the mounting element which creates the tension is proportional to the rotation angle. This proportion should be determined in advance because it is dependent on the assembly and not on the mounting element. The dispersion of the stress is improved in particular conditions due to the earlier trials and the geometric specifications of the engaging pieces which can be expensive. In addition the danger of entering into the plastic zone is not excluded because of the variability of the starting point of the measurement of the angle, in theory called prestress torque. It is to be noted that the prestress torque can be an accidental couple, for example from crossthreading; in this case the tension is far from being created.
Starting from the combination of these two measurements of torque and angle a tightening principle has been worked out: the permanent monitoring during the application of the torque of the variation of the gradient of the torque as a function of the angular advance allows one to establish an identifiable point of the function torque/angle, called the point of the elastic limit. This method puts the screw into a state of constraint (or of prestress) which uses all of the prestress possible of the assembly to the detriment of the service load. An advance determination of the tension is done in order to integrate the variations of section and the resistance to traction of the materials forming the mounting element. This method is very sensitive to the reaction forces of the means that apply the torque; for example flexion of the machine base, slipping of the holder of the assembled element. The point of the elastic limit can be that of the first element that breaks in the chain of mechanical action and reaction. In order to be applied with a maximum of safety, this method requires extra precautions that are expensive. This method offers a precision of .+-.10% in the tension of a single point using the entire capacity of the mounting element for the prestressing. The major disadvantage of this method is the penetration, even minimal, into the plastic zone of the mounting element. The trials of controlling and/or of monitoring of the slope (factor director of the tangent to the curve) at any point of the elastic zone are shown to be imprecise and varied. In this case one adds up the dispersion of the torque due to friction and the dispersion of the angle due to the flexibility of the assembly.
In order to eliminate variations of the stress due to diverse variations as discussed above, a new approach to tightening under stress is created by the ultrasound method.
This method is based on the variation of the time it takes traveling sound to move uniquely in the interior of the mounting element and it has nothing to do with all the other parameters not related to the element as well as the variations of the thickness of the pieces engaged together. The variation of the travel time is in theory directly related to lengthening. The relation of tension/lengthening is tied to the strength of the material, to its section, and to its starting length. This purely comparative method requires modelling (a specimen element) of the mounting element that will be tested for determining the variation of a travel time specific to a given stress. The measured lengthening is only a small part of the real length of the mounting element. In effect when one is working with the propagation of sound, only the stressed part of the mounting element takes part in the lengthening. This stressed part depends on the thickness of the engaged parts. Machining tolerances reflect directly on the elongation seen. The mounting element enters into these geometric dimensions mainly for the total variation of the travel time which depends on the length of the element relative to its stressed part. This variation is reflected in the travel time and in the variation in the ratio of length stressed to total length. In addition the parallelism of the planes of reflection of ultrasonic waves (essentially for a screw the head and the base of the stem) intervene directly in the effective position of the device measuring the travel time. The measurement of the lengthening should be done during the tensioning of the element because repositioning introduces errors that can double the degree of imprecision of the ultrasound method. Altogether these tolerances can be mastered with considerable expense; the precision of the effective measure of stress is similar to that of the elastic limit with the advantage of being able to be located anywhere of the admissible load of the mounting element with nonetheless the inconvenience of only being a probable indication of tension taking into account the reference to a sample (theoretical model) that is part of the average treatment of a lot of mounting elements.
The conditions of use of this method require important precautions with respect to connection of the measuring device, lack of markings on the head of the mounting element, actual temperature at which the measurement is taken, cleanness of the measurement surfaces, etc, precautions that entail high use costs and limited production speed. The cost of the ultrasound measuring device is itself very high, and this method can only be used in particular circumstances where the costs and the production rates are not a problem. This method is often retained as a means for monitoring or determining the conventional parameters of screwing (torque and angle).
In summary, none of the above-discussed methods provides a direct measurement of the tension, they are either comparative or theoretical and normally require advance trials to adjust the parameters and to know how the assembly will react. There is always at least one parameter that varies from one threaded element to another and which because of this creates a certain incertitude in the tension revealed:
Torque method: extreme variation in stress, stress unknown; PA1 Method using the angle: limited variation of the tension, advance estimated stress; PA1 Method using the elastic limit: One point of tension at the maximum of the capacity of the screw, Tension estimated in advance, The environment must be controlled; PA1 Ultrasound method: One point of stress and one point somewhere on the capacity of the mounting element, Tension estimated in advance, The machining tolerances and geometry must be controlled, Use limited by price. PA1 Eapp=energy applied to the mounting element on screwing-in PA1 Eapp'=energy applied to the mounting element on screwing-out, PA1 Etfr=energy converted into tension of the mounting element, PA1 Ett=energy consumed by the head or nut of the mounting element, PA1 Eflt=energy consumed by friction of the thread of the mounting element, PA1 Etg=energy consumed by friction of the stem of the mounting element. PA1 In taking the time derivative of equation (3) one gets: EQU d(Etfr)/dt=[d(Eapp)/dt-d(Eapp')/dt]/2 at time t Ptfr=(Papp-Papp')/2(4) PA1 Papp=power applied for screwing-in, PA1 Papp'=power applied for screwing-out, PA1 Ptfr=power converted into stress PA1 Capp=torque applied for screwing-in PA1 Capp'=torque applied for screwing-out and reciprocally EQU Ft=(capp-capp')/(P/.pi.) (7) PA1 1. Speculation about the constant of friction (which varies in reality at each point of advancement as a function of the constant area, the condition of the surface, and the contact pressure) because the for the proportionality constant (K). This is friction constant varies it is impossible to predict the final screwing-in torque at a point different from that of the measurement or even less the stress existing at this torque. PA1 2. The screwing-in/screwing-out torques should be measured under particular conditions which are not detailed in the cited document and in the absence of this the measurements vary widely, being able to range to a negative tension ! (ex; a screwing-out torque greater than a screwing-in torque caused by an adherence or gluing). There is an obligation to measure right from the start of the movement of the threaded element and/or to not read torques except at precise times as proposed by the present invention. PA1 3. Absence of the notion of correlating positions to the places where the torques are read on screwing-in and screwing out which for a progressive force like the increase in torque and in tension introduces an offset of the supplemental torque which seriously hurts the method (detorqueing of the threaded element). PA1 4. With respect to the modalities of working according to the claims 5 and 6 of document EP-A-0,096,620, introducing the notion of a "gradient," it is to be noted that the way of carrying out the method suffers from the main defects as those described earlier and more that the taking of a derivative increases greatly the anomalies of measurement PA1 the tractive force applied to the mounting element, PA1 the compressive force exercised by the mounting element, or PA1 the tensile force inside the mounting element. PA1 Ft=tension force in Newtons PA1 Cv=torque at the limit of screwing-in in Newton-meters PA1 Cd=maximum torque on screwing-out in newton-meters PA1 .pi.=circle constant PA1 P=pitch of the screwthread in meters PA1 For Ft in Newtons PA1 K=(P/.pi.).multidot.S PA1 where S=12, if Cv and CD are expressed in Newton-feet PA1 (Nft) and P in inches (in) PA1 where S=1.6584, if CV and CD are expressed in PA1 poundal-feet (pdl-ft) and P in inches (in) PA1 Ft=tension force in Newtons PA1 Ev'=an element representing the energy at the limit of screwing-in PA1 Ed'=an element representing the maximum screwing-out energy PA1 .tau.=a constant including the .pi. circle constant times the motor efficiency for the type of energy PA1 P=the pitch of the screwthread in meters PA1 the tractive force applied to the mounting element, PA1 the compressive force exercised by the mounting element, or PA1 the tensile force inside the mounting element at the start of the operation. PA1 When the motor means is provided with a torque meter, the directly relationship is expressed as follows: EQU Ft=(Cv-Cd).multidot.I/K where k=P/.pi. and I=Cg/Cv PA1 Ft=tension force in Newtons PA1 Cg=torque at the start of screwing-in in Newton-meters PA1 Cv=torque at the limit of screwing-in in Newton-meters PA1 Cd=maximum torque on screwing-out in Newton-meters PA1 .pi.=circle constant PA1 P=pitch of the screwthread in meters PA1 Ft=tension force in Newtons PA1 Eg'=an element representing the energy at the start of screwing-in PA1 Ev'=an element representing the energy at the limit of screwing in PA1 Ed'=an element representing the maximum screwing-out energy PA1 .tau.=a constant including the .pi. circle constant times the motor efficiency for the type of energy PA1 P=the pitch of the screwthread in meters PA1 to the rest limit if there is no inertia, or PA1 to the start of sliding if there is inertia in order to restore the initial conditions of the assembly while having monitored the existing tensile force in the mounting element or the tractive force to which the mounting element is subjected or even the compressive force exerted by the mounting element. PA1 the traction force to which the mounting element is subjected; PA1 the compression force exerted by the mounting element, or PA1 the tension force in the mounting element, these steps being followed by either PA1 the action of rescrewing-in of the mounting element to a position whose measured force value is equal to the force necessary for the assembly, or PA1 the traction force to which the mounting element is subjected; PA1 the compression force exerted by the mounting element, or PA1 the tension force in the mounting element. PA1 to fix the starting point of the increase of rotation force (compensating for play) on the measured position by the sudden increase in the value of the directing factor; PA1 to cancel out the sum of torsions for screwing-in/screwing-out while subtracting for each of these actions the value of the effective position relative to the value of the rotation force from the value of the measured position before the mounting element started rotation,, the effective position being obtained by the sudden decrease in the value of the directing factor; PA1 to cancel out the sum of torsions of screwing-out/rescrewing-in while subtracting for each of these actions the value of the effective position relative to the value of the rotation force from the value of the measured position before the mounting element started rotation, this position value being calculated during rescrewing-in in the proportion of the applied rotation force, the effective position being obtained by the sudden decrease in the value of the directing factor; PA1 for evaluating at any instant during the act of screwing-in/screwing-out or screwing-out/rescrewing-in the value of torque relative to the applied rotation force. PA1 screwing-in with a rotation force formed by a variable value and a fixed "delta" value; PA1 subsequently screwing-out by a value equal at most to the variable value, the screwing-out being able to be partial; PA1 repeating the action of screwing-in/screwing-out while increasing the variable value as long as screwing-out is possible, the progression of this variable value being able to be equal to the difference between this variable value and the value realized during screwing-out or a fraction of this difference so as to moderate the action while insuring a rapid convergence of this action toward a condition of it being impossible to screw out; PA1 the traction force to which the mounting element is subjected; PA1 the compression force exerted by the mounting element, or PA1 the tension force in the mounting element where, when the motor means is provided with a torque meter, the direct relationship of this rotation-force value is expressed as follows: EQU .delta.C=FT.multidot.K where K=P/.pi. PA1 .delta.C="delta" torque in Newton-meters PA1 Ft=tensile force in Newtons PA1 .pi.=circle constant PA1 P=pitch of the screwthread in meters PA1 .delta.E'=a factor representing energy PA1 Ft=tensile force in Newtons PA1 .tau.=a constant including the circle constant .pi. times the efficiency of the motor in converting energy PA1 P=pitch of the screwthread in meters PA1 Ctfr=(Ft.multidot..pi.)/2P torque converted PA1 .mu.=Ctfr/Capp general efficiency on screwing-in PA1 .mu.'=Ctfr/Capp general efficiency on screwing-out PA1 cs=1/.mu.' safety coefficient PA1 Capp=Ctfr+Ctt+Cflt+Ctg general distribution of screwing in PA1 CappO=Ctfr+Ctt screwing in (rest limit) PA1 -CappO'=Ctfr-Ctt screwing out (rest limit) PA1 Ctt=(CappO+CappO')/2-Ctfr head friction torque PA1 Cappl=Ctfr+Ctt+Cflt+Ctg screwing in (screwthread rotation) PA1 -Cappl'=Ctfr+Ctt+Cflt-Ctg screwing out (screwthread rotation) PA1 cflt=((Cappl+Cappl')/2-Ctt screwthread friction torque with negligible stem friction PA1 Cflt=((Cappl+Cappl')/2-Ctt-Ctg screwthread friction torque with high stem friction at high tension PA1 .mu.n=coefficient of friction PA1 Cx=friction torque (Ctt, Cflt, etc.) PA1 Ft=tension force of the method PA1 dI=inside diameter of the contacting surface PA1 d2=outside diameter of the contacting surface
The more efforts are made to precisely estimate the stress, the more the apparatus for the procedures costs and the more the procedures cost to do and use. Having to end up with an estimate rather than a direct measurement brings with it further monitoring costs. Industrial requirements concerning the new methods clearly indicate the inadequacy of the current ones.
Knowing that the methods described above all have one or more inadequacies, a new method has been proposed based on the study of several hundreds of screw connections and all sorts of mounting elements of the difference between the torque level applied during screwing-in and the torque value applied during screwing-out, which leads to the simplest of applying stress in order to discover the energy relationships between the torque and the energy of the screw assembly to arrive at a new process.
In effect the screwed assembly is formed of elastic materials. From this it therefore acts like a spring. Stretching this spring in its elastic range by means of a force Ft stores in this screwed assembly an energy E which can be expressed as follows: EQU E=(Ft .delta.L)/2 (1)
where .delta.L is the difference between the final length and the starting length of the mounting element.
This accumulated energy (1) is recovered during screwing-out because of the reversibility of the mounting element because of its helicoidal thread and the potential energy residing in the screwed assembly.
If this lengthening is done by converting a torque Cu into a tractive force by means of a helicoidal ramp of angle .alpha. developed on a circumference of diameter 2.multidot.r, this expression becomes: EQU E=(Cu.multidot.cot.alpha./r).multidot.(.delta.L/2) with cot.alpha.=2 .pi.r/ P EQU P being the pitch of the screwthread so that E=Cu.multidot..delta.L.multidot..pi./P (2)
This stored energy is not apparent and only represents a fraction of the applied energy because of the energy consumed by friction which mainly impedes spontaneous screwing-out of the mounting element. This friction is the same during screwing-in and screwing-out and is created by the parts of the contact surfaces of the different elements forming the assembly. From this fact it can be seen that there is a difference between the energy applied during screwing-in and screwing-out due to the energy that is stored up during screwing-in and recovered with screwing-out when the energy consumed by friction is constant.
The general distribution of the different energies for an energy applied are described as follows: EQU during screwing-in Eapp=Et fr+Ett+Eflt+Etg EQU during screwing-out -Eapp'=Et fr-Ett-Eflt-Etg hence Etfr=(Eapp-Eapp')/2(3)
wherein
wherein
and taking account of the generally relationship Pu=C. (power=torque.times.rotation speed) equation (4) simplifies to EQU Cu=(Capp-Capp')/2 (5)
The useful torque or torque transformed into tension Cu can be described according to (2) as a function of the converted energy as follows: EQU Cu=Etfr.multidot.P/(.pi..multidot..delta.L)
inserting (1) one gets: EQU Cu=FT.multidot.P/(2.pi.)
wherein
In other words, the stress force (in traction or compression) can be monitored as a function proportional to the difference between the torque applied on screwing-in, especially the maximum torque at the stopping point, and the torque applied on screwing-out, especially the maximum screwing-out torque, the difference being divided by (P/.pi.), a quantity proportional to the pitch of the screwthread. The quantity (P/.pi.) constitutes a constant for a given thread which allows the immediate conversion of the difference between the torques of the act of screwing-in then of screwing-out a threaded mounting element, expressed in Newton-meters (Nm) and a stress expressed in Newtons (N) whose precision is directly derived from the precision of the torques.
From the formula (7) one can deduce the general definition for a method of monitoring and controlling tension or compression of a threaded element in which the tensile or compressive force (Ft) of the threaded element is determined by the action of subtracting the torque applied on screwing-out (Capp') from the torque applied on screwing-in (Capp) and dividing this difference in torques by a factor proportional to the pitch (p) of the screwthread of said element. Such a process is generally and theoretically described by document EP-A-0,096,620.
In any case this document does now show a workable and reliable operational mode for putting this method into industrial use; the teachings of document EP-A-0,096,620 only allow an approximation of the screwing-in force exerted by the threaded element and thus the disadvantages of the other prior-art techniques are not avoided.
More precisely an analysis of document EP-A-0,096,620 allows one to discern, for the process that it describes, on one hand the theoretical insufficiencies and on the other hand the technical insufficiencies.
The mention of a factor of proportionality between the screwing-in force and the difference of the screwing-in/screwing-out torques seen at a particular point based on document EP-A-0,096,620 has several problems:
The preceding points are confirmed by the content of document EP-A-0,096,620 which admits that the factor K is different for several tries from which there comes the need to evaluate it statistically but which does not resolve the problem of the proportionality of the factor K over all of the screwing-in process. At best one gets an approximation of K at one particular point relative to an angular position.
In addition to the theoretical deficiencies developed above, document EP-A-0,096,620 has no practical description of the means which allow one to apply the theory developed in this document, and its sole figure is limited to the representation of a classical threaded assembly.
In view of this state of the art, the present invention is aimed at furnishing a perfected method while giving a specific operating mode allowing good reproducibility and better precision.
In this method of monitoring and controlling the tension or compression of a threaded element where the tension or compressive force of the threaded element is determined by subtracting the unscrewing torque from the screwing torque and where the difference between these torques is divided by a factor proportional to the pitch of the screwthread of said element, the invention provides that these torque values are measured either statically at the rest limit during screwing-in and screwing-out or dynamically at a succession of corresponding positions during screwing and unscrewing.
Simple means allow one to carry out the method whose general definition was just given, for learning, controlling, or monitoring screw assemblies in an inexpensive and more efficient manner than any of those now known and thus the invention allows one to use threaded mounting element of lesser cost and of lower quality while being certain of the holding of these mounting elements.
A first way of carrying out the method according to the invention consists of applying the process to a measurement of tension by using a manual, mechanical, pneumatic, hydraulic, or electric motor capable via a coupling device connected to the threaded mounting element of the assembly of applying a controlled force for rotating this mounting element.
In the case of negligible mechanical inertia, the method is characterized by the action of screwing in with the aim of taking at the rest limit of sliding of the movable part of the mounting element on the fixed part of the assembly the value of the rotatory force, then by the action of screwing out with the aim of taking, at the rest limit during the sliding of the movable part of the mounting element on the fixed part of the assembly the value of the rotatory force as it goes through its maximum, then of taking the difference between these two force values to divide them by a factor that is mainly proportional to the pitch of the screwthread of the mounting element so as to obtain a value representing in precise limits either:
When the motor means i provided with a torque meter the direct relationship is expressed as follows: EQU Ft=(Cv-CD)/k where k=P/.pi.
wherein
The indirect relation is established for the expression of the tension force in another system of measure by a supplementary factor establishing agreement between the units of force and the units of length, for example:
When the energy source of the motor is monitored the direct relationship is expressed as follows: EQU Ft=(Ev'-Ed')/K where K=P/r
wherein:
The element representing the energy can be any element giving the image of the torque of the motor means (current, voltage, pressure, etc.) multiplied by a constant or function f(x), but also the torque of a mechanical means using the flexion or the torsion of an element for which the conversion is not generally a constant but can also be a function (x).
In the case of significant mechanical inertia the process is characterized by the action of screwing-in while intending to obtain just as the movable part of the mounting element starts to slide on the fixed part of the assembly ,the value of the rotation force and then of obtaining at the rest limit of the sliding of the movable part of the mounting element on the fixed part of the assembly the value of the rotation force, then dividing so as to obtain at the rest limit during the sliding of the movable part of the mounting element on the fixed part of the assembly the value of the rotation effort as it goes through its maximum, then of taking the difference of these two force values to divide this result by a factor mainly proportional to the pitch of the screwthread of the mounting element so as to obtain a value representing these precise limits either:
wherein:
The indirect relationship is established for the expression of the tension force in another system of measure by a supplementary factor establishing agreement between the units of force and the units of length.
When the energy source of the motor means is monitored, the direct relationship is expressed as follows: EQU Ft=(Ev'-Ed')I/K where K=P/r and I=Eg'/Ev'
wherein:
The element representing the energy can be any element giving the image of the torque of the motor means (current, voltage, pressure, etc.) multiplied by a constant or function f(x), butalso the torque of a mechanical means using the flexion or the torsion of an element for which the conversion is not generally a constant but can also be a function (x).
The indirect relation is established for the expression of the tension force in another system of measure by a supplementary factor establishing agreement between the units of force and the units of distance.
A nondestructive monitoring of the assembly is done by the action described above of measuring tension followed by the action of rescrewing-in up to the force first used, more specifically either
According to a mode of carrying out the process according to the invention one carries out a simple control by successive approximations (iterative process). To this end on a mounting element that is already stressed in the known manner one exerts the above-defined action as many times as necessary until the ratio of the necessary tensile force to the existing tensile force is equal to 1. In order to limit the amplitude of the screwing-in/screwing-out, the value of the screwing-in/screwing-out force can be modified by a factor (lessening) able to assure the convergence of the action toward the tension necessary in a number of strokes defined by the precision defined in the control of the action.
One can furthermore according to the invention carry out a "dynamic control" by using a manual, mechanical, pneumatic, hydraulic, or electric motor means capable via a coupling member connected to the threaded mounting element of insuring a controlled rotation force and detection of the actual angular position of the mounting element.
With the mounting element not tightened one tightens the mounting element while at regular intervals spaced according to the desired precision and corresponding to respective angular positions one ascertains the force applied and then one loosens the mounting element while at the same positions one determines the force applied for screwing-out, one takes the difference between the screwing-in and screwing-out force for each position and divides it by a factor proportional to the pitch of the mounting element so as to obtain a list of values representing for each of the positions
the action of rescrewing-in the mounting element to a screwing-in force corresponding to the position at which one measured a force value equal to the force necessary for the assembly.
This action of rescrewing-in can be modified with respect to the position chosen and to the value of the force in order to anticipate the inertia of the motor means used. This modification can be a constant held in memory and corresponding to a predetermined rotation force or deducted from the stop phases of said means when one samples different force values of rotation of the mounting element.
This action of rescrewing-in can be corrected with respect to the position chosen and to the value of force in order to integrate the torque of the motor means mainly when the position detector is not fixed to the mounting element or to the coupling member. This correction can be taken from a correction table held in memory and deducted from the torque curve, that is from the relationship of the rotation force and the effective position.
The agreement of the positions of screwing-in and scre- wing-out can be done by correlating the position of maximum screwing-in force with the position of maximum screwing-out force while abstracting in this case the idea of a torque curve.
The actions of screwing in in order to obtain, at regular intervals of the value of the rotation force determined by the desired precision, the successive positions of the mounting element then of screwing out in order to obtain, at regular intervals for the same values, the successive positions of the mounting element, allows interpolating between values.
The fact of establishing a function relative to the rotation force with respect to the position of the mounting element, instead of establishing a list of values mainly when the control is done by a calculator, a processor, or a microcontroller in order to resolve by calculation the position or the rotation force necessary for the assembly, belongs to the same type of execution.
Nondestructive control of the assembly is done by using a manual, mechanical, pneumatic, hydraulic, or electric motor means capable via a coupling member connected to the threaded mounting element of insuring a controlled rotation force and detection of the actual angular position of the mounting element. With the mounting element that is already tightened one screws out the mounting element while at regular intervals spaced according to the desired precision and corresponding to respective angular positions one ascertains the force applied and then one retightens the mounting element while at the same positions one determines the force applied for screwing-out, one takes the difference between the screwing-out and rescrewing-in force for each position and divides it by a factor proportional to the pitch of the mounting element so as to obtain a list of values representing for each of the positions either
This action of rescrewing-in can be modified with respect to the position chosen and to the value of the force in order to anticipate the inertia of the motor means used. This modification can be a constant held in memory and corresponding to a predetermined rotation force or deducted from the stop phases of said means when screws out.
This action of rescrewing-in can be corrected with respect to the position chosen and to the value of force in order to integrate the torque of the motor means mainly when the position detector is not fixed to the mounting element or to the coupling member. This correction can be taken from a correction table held in memory and deducted from the torque curve, that is from the relationship of the rotation force and the effective position.
When in the above the agreement of the positions of screwing-in/screwing-out or screwing-out/screwing-in does not work because of the torque of the motor used and the different deformations in the reaction chain of motor motor/housing/assembly, mainly when the position detector is not fixed on the mounting element or the coupling member relative to the assembly and because of the additive effect of play in the transmission with the rotation force, agreement of the positions of screwing-in/screwing-out is done by means of a "torque curve."
The position detector sees the position of the mounting element through an assembly which deforms proportionally to the rotation force applied by the motor means. So long as the resistant torque is not exceeded by the motor torque, the mounting element does not start rotating while the position detector records a displacement proportional to the applied torque. The effective position is masked by an apparent position due to the formation of the reaction chain. The torque curve is set up starting from the relationship of the value of the rotation force to the value of the observed position. The tangent to this curve is established by the variation of the value of the rotation effort relative to the variation of the value of position. The sudden change of the directing factor of the tangent to this curve, when the mounting element actually rotates, gives the precise position when the actual rotation starts. The resisting torque of the mounting element is the result of different contacting surfaces, head or nut, stem, screwthread. The element is subjected permanently (when it is stressed) to torque due to the tensile force on the helicoid of its screwthread. The modification of the tensile force is effected essentially when the resisting torque created by the screwthread is overcome. Rotation of the entire mounting element only starts at the instant when the screwthread advances. There is thus a difference between the start of rotation of the head or of the nut and the actual rotation of the mounting element. This difference in action is effective as a supplemental torque or a partial detorqueing of the mounting element depending on whether it is being screwed in or out. This torque variation of the mounting element affects the value of the recorded position in particular when the length of the mounting element is large with respect to its cross section. This variation creates a difference in slope between screwing in and out.
An analysis of the curve by the control and/or monitoring system makes it possible to work with great finesse, according to the desired precision in correlating the actual positions between screwing in and out to synchronize them with the applied rotation forces, namely:
The above described "dynamic control" action establishes a list of values of rotation efforts as a function of position intervals. The relationship between the difference of a value V and a value V-1 on the corresponding position interval gives the directing factor of the curve at a given interval. The succession of differentiation of the values such as V+1 and V, V+2 and V+1 . . . V(n) and V(n-1) where n represents the row of value in the list, provides a list of directing factors that are used in the variation of their value:
It is also possible according to the invention to carry out a "floating control" allowing by using a manual, mechanical, pneumatic, hydraulic, or electric motor means capable via a coupling member connected to the threaded mounting element of insuring a controlled rotation force, with the following succession of screwing-in and -out actions:
the "delta" force value multiplied by a factor proportional to the pitch of the screwthread of the mounting element representing either:
wherein:
where, when the energy consumption of the motor means is monitored the direct relationship of this rotation-force value is expressed as follows: EQU .delta.E'=Ft.multidot.K where K=P/.tau.
wherein:
The element representing energy can be any element giving the image of the torque of the motor means (current, voltage, pressure, etc.) multiplied by a constant or function f(x), but also the torque of a mechanical means using the flexion or the torsion of an element for which the conversion is not generally a constant but can also be a function (x).
The motor means can only be monitored in one way or differentially. The monitoring of the absolute rotation force is not indispensable.
Finally the process according to the invention allows one to study the coefficients of friction and the general efficiency of the mounting element in the assembly:
Through the principle of the invention and dynamic control one establishes a precise relationship between the torque applied on screwing in or out and the tension that exists in the mounting element. This tension allows one at any time to calculate the torque effective converted to produce this same tension. The relationship of this converted torque to the applied torque fixes the efficiency of the mounting element.
The efficiency of the mounting element is the resultant of the frictions of the different contacting surfaces, heat or nut, stem, screwthread. Their distribution is unknown. The usage of the "torque curve" allows each of the torques caused by friction of the contacting parts to be isolated by different rotations of each part of the mounting element seen by the torque curve.
In the study of friction coefficients when the measurement of the change in positions is not sufficiently demarcated (the case with short screws) extra weight is given in the equations of friction to the torque caused by friction of the screwthread without changing its value by the introduction under the head or nut of an intermediate element, such as an abutment bearing or other element with known or minimal coefficient.
From the current relationship :.mu.n=Cx/[Ft.multidot.(dI+d2)] for bodies of revolution, one deducts the coefficient of friction starting with the distribution of torques for each torque Cs as a function of the tension Ft and of the geometry of the mounting element wherein in this relationship